Saturated molecules of period 2 p-block atoms, such as CH4, NH3, and H2O with full octets, are not expected to dimerize by covalent bond formation. A computational examination of NH3 as an exception is carried out. Quite high in energy relative to the isolated monomers, the covalently bound local singlet hexahydridodinitrogen (N2H6) minimum of D3d symmetry on the potential energy surface of the ammonia pair is probed. The structure and bonding of this double-octet dimer system are confirmed, but we find that the location of the species on the potential energy surface is highly sensitive to model chemistry, especially for post-Hartree-Fock methods. Of several singlet dimer pairs examined herein, no other system with period two p-block central atoms formed stable covalent minima. The prospect for dissociation through competing channels provides strong reasons for pessimism about any realization of the N2H6 species. Yet, the observed energy lowering for N2H6, which is strengthened by Rydberg bonding, challenges chemists to continue pressing against the perceived limits of chemical bonding.
Donald et al. (Mon,) studied this question.